This is a very neat video of salt on a vibrating surface. The surface is being excited by a transducer of some sort across a range of audible frequencies. The salt reacts to this energy. The pattern of the salt on the surface illustrates the nodal breakdown of the surface into various harmonics. That is, as the frequency increases the surface changes from moving as a single piston to breaking down into ever smaller “nodes.” This is an excellent visual example of resonance in action.
This link came to me by way of the Twitter feed from The Art And Science of Sound, a project lead by master recording engineer and musician Alan Parsons. The project is a DVD set explaining the many mysteries of modern recording studio techniques.